1. Field of the Invention
This invention relates to a method of making an aluminum alloy plate for bearing by cladding a bonding layer comprising a pure aluminum or an aluminum alloy excluding Sn onto a bearing alloy layer comprising an aluminum alloy.
2. Description of the Prior Art
In making a bearing lined with an aluminum alloy (aluminum alloy bearing), a bearing alloy layer comprising an aluminum alloy containing Sn is generally bonded to a back metal with a bonding layer being interposed therebetween, so that a bimetal is made. The bonding layer comprises a pure aluminum or an aluminum alloy. The bimetal is then machined into an aluminum alloy bearing. The bonding layer is bonded to the bearing alloy layer prior to bonding of the bearing alloy layer to the back metal. The bonding layer is cladded onto the bearing alloy layer when bonded thereto. More specifically, a plate formed into the bearing alloy layer and a plate formed into the bonding layer are passed between a pair of flat rolls so that the plates are rolled down at a large reduction ratio such that the bonding layer is cladded onto the bearing alloy layer.
The aluminum alloy bearing is generally used for engines of automobiles or industrial machines since it has excellent fatigue resistance, wear resistance, etc. However, with recent advancement in the performance of engine, the bearing has necessitated a further improvement in the fatigue strength, wear resistance, etc. The bearing aluminum alloy has been changed from a relatively soft Alxe2x80x94Sn system to a hard Alxe2x80x94Snxe2x80x94Si system or Alxe2x80x94Snxe2x80x94Sixe2x80x94Mn system in order that the requirement may be met.
However, the bearing aluminum alloy of hard Alxe2x80x94Snxe2x80x94Si system or Alxe2x80x94Snxe2x80x94Sixe2x80x94Mn system has a low rolling workability of the aluminum alloy. Accordingly, there is a possibility that crack may occur when the bonding layer is cladded onto the bearing alloy layer. The crack occurs in both widthwise edges of the aluminum alloy plate formed into the bearing layer.
When a bonding layer plate is cladded on a bearing alloy layer plate so that both are bonded together, a higher bonding strength is ensured as the reduction ratio becomes high. Accordingly, the bonding layer plate is cladded on the bearing alloy layer plate at a reduction ratio of 40 to 50% when a relatively soft Alxe2x80x94Sn alloy is used for the bearing alloy layer. The reduction ratio is defined as [(plate thickness prior to rollingxe2x80x94plate thickness after rolling)/plate thickness prior to rolling]xc3x97100.
However, the Alxe2x80x94Snxe2x80x94Si alloy or Alxe2x80x94Snxe2x80x94Sixe2x80x94Mn alloy, each of which serves as the bearing alloy layer and has a low rolling workability, cannot withstand pressurization or cladding at such a high reduction ratio as mentioned above in order that occurrence of crack may be prevented for increase in the yield. Accordingly, since the bonding layer is cladded onto each layer of these bearing alloys at a low reduction ratio, the bonding strength between the bearing alloy layer and the bonding layer becomes insufficient.
Therefore, an object of the present invention is to provide a method of making an aluminum alloy plate for bearing in which even when an aluminum alloy having a low rolling workability is used for the bearing alloy layer, the bonding layer can be cladded onto the bearing alloy layer at a high reduction ratio while occurrence of crack is prevented, so that an aluminum alloy plate for bearing having a sufficient bonding strength can be made.
To achieve the object, the present invention provides a method of making an aluminum alloy plate for bearing which is made by cladding a bonding layer comprising a pure aluminum or an aluminum alloy excluding Sn onto a bearing alloy layer comprising an aluminum alloy. The method comprises the steps of fitting a concave portion of a first roll in a convex portion of a second roll, the first roll having both axial ends with large diameter portions respectively, the second roll having both axial ends with small diameter portions respectively, and passing superposed plates formed into the bearing alloy layer and the bonding layer respectively through a roll gap defined between the concave and convex portions and closed by the large diameter portions of the first roll so that the plates are rolled down at a reduction ratio of not less than 50% while both widthwise ends of each plate is restricted by the large diameter portions of the first roll respectively such that the bonding layer is cladded onto the bearing alloy layer.
According to the above-described method, the axial large diameter portions of the first roll closing both ends of the roll space restrict the widthwise ends of each plate, thereby preventing widthwise extension of each plate. Consequently, since occurrence of crack is reduced, both layers can be rolled down at a high reduction ratio of not less than 50% even when the aluminum alloy having a low rolling workability is used for the bearing alloy layer, whereupon the bonding strength between the plates can be increased.
In a first preferred form, a cast plate quenched at 3 to 6xc2x0 C./sec. by a belt casting machine is used as the plate made into the bearing alloy. The belt casting machine performing casting between a pair of endless belts is known in the art. The known belt casting machine has a casting space defined to be horizontal or slightly inclined between substantially horizontal portions of the belts. The belts are driven to travel while being cooled. A molten metal is supplied into the casting space and cooled by the belts to be solidified into the shape of a plate. The solidified metal is then fed out of the casting space continuously.
The aforesaid belt casting machine of the movable mold type has a higher casting speed and productivity than continuous casting machines of the fixed mold type. Accordingly, even bearing manufacturers employ belt casting machines to cast aluminum alloy plates for a bearing alloy layer. However, the conventional belt casting machines have low cooling rates such that a cast plate is gradually cooled. As a result, since crystals become easy to coarsen or segregate, the rolling workability and bearing characteristic are deteriorated.
In view of the above-described problem, the inventors developed a belt casting machine with a water sprayer. A cast plate fed out of the casting space is quenched at a cooling rate of 3 to 6xc2x0 C./sec. by water sprayed from the water sprayer so that the crystals are prevented from coarsening. However, since the machine carries out the belt casting method although being provided with the water sprayer, the casting speed becomes high but the cooling rate becomes low. Accordingly, it is difficult to completely prevent the crystals from being coarsened. Thus, the bearing alloy plate made by the belt casting machine with the water sprayer fairly improves a rolling workability but is still difficult to roll. Particularly when the plate contains Si, the cladding using ordinary flat rolls results in occurrence of crack on the widthwise edges of the plate.
The bonding layer can be cladded onto the bearing alloy plate cast by the belt casting machine by the method of the present invention without occurrence of crack, although the plate is fairly difficult to roll. In the casting by the above-described belt casting machine, Sn and Si segregate and an intermetallic compound with aluminum coarsens or segregates when the cooling rate is below 3xc2x0 C./sec. As a result, the plastic workability such as rolling workability is reduced such that the fatigue resistance and wear resistance both as the bearing characteristics become unstable. When the cooling rate exceeds 6xc2x0 C./sec., quenching results in segregation on the surface of the plate, whereupon milling the surface of the plate becomes difficult.
In the present invention, the following two novel aluminum alloys are particularly suitable as the bearing alloy layer on which the bonding layer is cladded. One is a novel aluminum alloy comprising, by mass, 3 to 40% Sn, 0.5 to 7% Si, 0.05 to 2% Fe, and balance of Al and unavoidable impurities and a ternary-element intermetallic compound of Alxe2x80x94Sixe2x80x94Fe is crystallized. The other is a novel aluminum compound comprising, by mass, 3 to 40% Sn, 0.5 to 7% Si, 0.05 to 2% Fe, at least one or more of Mn, V, Mo, Cr, Co, Ni and W in an amount or a total amount of 0.01 to 3%, and balance of Al and unavoidable impurities and a multi-element intermetallic compound of Alxe2x80x94Sixe2x80x94Fe containing said at least one or more of Mn, V, Mo, Cr, Co, Ni and W, is crystallized.
The technical background of the development of the aforesaid novel aluminum alloys will now be described. With recent development of high performance engines, engine bearings necessitate further improvement in the fatigue strength and wear resistance. Regarding the fatigue strength, elements such as Cu, Mn and V are added to the aluminum alloy to strengthen the latter. For the purpose of improvement in the wear resistance, JP-A-58-64332 discloses that Si is added to the aluminum alloy and the size and distribution of Si particles crystallized in the aluminum alloy are controlled. Further, JP-A-58-67841 discloses that Mn, Fe, Mo, Ni, etc. are added to the aluminum alloy so that an intermetallic compound between Mn etc. and Al is crystallized in the aluminum alloy. These two cases propose an improvement in the conformability and anti-seizure property of the aluminum alloy, thereby improving the wear resistance.
The above-noted JP-A-58-64332 and JP-A-58-67841 disclose that a desired effect can be achieved when the sizes of Si particles and the intermetallic compound range between 5 xcexcm and 40 xcexcm, respectively. Generally, hard particles contained in Al are uniformly distributed to be used for strengthening the aluminum alloy, and the effect is larger as the size of particles becomes small. In the aforesaid two cases, however, when the size of Si and the intermetallic compound are controlled so as to range between 5 xcexcm and 40 xcexcm, the strength of the Al matrix and accordingly the fatigue strength of the Al alloy are reduced as the size of Si and the intermetallic compound is relatively large. Thus, the anti-seizure property cannot be improved when crystallized particles are rendered small for improvement in the fatigue strength. On the other hand, the fatigue strength cannot be improved when the crystallized particles are rendered large for improvement in the anti-seizure property and accordingly in the wear resistance.
The inventors developed an Al alloy by crystallizing a ternary intermetallic compound of Alxe2x80x94Sixe2x80x94Fe or a multi-element intermetallic compound containing Alxe2x80x94Sixe2x80x94Fe as the base. The Al alloy can improve the anti-seizure property and wear resistance without reduction in the fatigue strength. The ternary intermetallic compound of Alxe2x80x94Sixe2x80x94Fe and the multi-element intermetallic compound containing Alxe2x80x94Sixe2x80x94Fe as the base are exceedingly stable, and its basic shape is not changed even by the heat treatment after cladding with a back metal. More specifically, Si crystallizes as a eutectic in the form like a three-dimensionally connected coral. The crystallized Si is crushed to pieces by rolling after casting or rolling in the cladding with the back metal. Further, Si also changes its form by a subsequent heat treatment. This is a characteristic of Si and particularly in the heat treatment in which the temperature exceeds 300xc2x0 C., Si changes into a relatively rounded so that a surface tension thereof is reduced. This tendency is enhanced in a material containing a large amount of Sn, for example, an Alxe2x80x94Sn bearing alloy.
However, the aforesaid ternary intermetallic compound or multi-element intermetallic compound does not change its crystallized form (an example is shown in FIG. 4) and does not change its form at a temperature for a usual heat treatment. Further, the ternary or multi-element intermetallic compound is crushed in the rolling step with plastic deformation or the cladding step during manufacture of the bearing. However, as the result of crush, the intermetallic compound takes a form with a sharp edge such as a broken piece of an edged tool. FIG. 5 shows an example of such a form. Although Si particles are rounded and broken into pieces through the steps of rolling and heat treatment, the aforesaid ternary or multi-element intermetallic compound retains an aggressive form with a sharp edge.
The ternary or multi-element intermetallic compound has a lapping effect on an associated shaft even when its amount is small. Particularly, the ternary or multi-element intermetallic compound stabilizes the relationship between the shaft in an unstable initial operation and the bearing. Thus, the ternary or multi-element intermetallic compound is effective in improving the conformability. More concretely, the ternary or multi-element intermetallic compound scrapes off protrusions on the surface of the shaft and an edge such as burrs around nodular graphite on the surface of the shaft. The ternary or multi-element intermetallic compound further prevents the Al alloy from wear due to adhesion to the shaft, which is a disadvantage of the Al alloy. Additionally, the ternary or multi-element intermetallic compound further scrapes away an adherent matter to thereby prevent seizure due to the adherent matter. Moreover, the ternary or multi-element intermetallic compound is relatively large even after the rolling step. Minutely pulverized Si particles are distributed in the Al matrix, thereby improving the strength of the Al matrix. Consequently, both improvement in the wear resistance and anti-seizure property and improvement in the fatigue strength can be achieved.
The reasons for the amount limitation of each novel Al alloy will be described below.
(1) Sn (3 to 40 mass %)
Sn improves surface properties such as anti-seizure property, conformability and embeddability as a bearing. When the Sn content is less than 3%, the above-mentioned effects are small. When it exceeds 40%, mechanical properties of the bearing alloy are deteriorated with the result of reduction in the bearing performance. A preferable Sn content ranges between 6 and 20%.
(2) Si (0.5 to 7 mass %)
Si dissolves in the aluminum matrix and partially crystallizes as a single substance of silicon particle to disperse finely, so as to enhance the fatigue strength of the material and serve to improve the anti-seizure property and wear resistance. On the other hand, Si is an essential element in order to form the Alxe2x80x94Sixe2x80x94Fe intermetallic compound and improves the lapping effect, anti-seizure property, and wear resistance. When the Si content is less than 0.5%, Si dissolves into the Al matrix such that the above effects are small. When it exceeds 7%, its crystal is coarsened, so as to reduce the fatigue strength of the bearing alloy. A preferable Si content ranges between 2 and 6%.
(3) Fe (0.05 to 2 mass %)
Fe crystallizes mainly as the Alxe2x80x94Sixe2x80x94Fe intermetallic compound, so as to produce the above-described effects. The intermetallic compound containing Fe prevents seizure with a shaft and improves the wear resistance. The characteristic is effective when the Fe content ranges between 0.05 and 2%. When the Fe content is less than 0.05%, the above-mentioned effects are small. When the Fe content exceeds 2%, the compound is coarsened and the bearing alloy becomes brittle, whereupon the rolling work causes trouble. A preferable Fe content ranges between 0.07 and 1%.
(4) Mn, V, Mo, Cr, Co, Ni, and W (at least one or more in an amount or a total amount of 0.01 to 3 mass %)
These are optional elements which constitute the multielement intermetallic compound in the present invention. More specifically, when a selected element a is added to Alxe2x80x94Sixe2x80x94Fe, a multi-element intermetallic compound of Alxe2x80x94Sixe2x80x94Fe-xcex1 is produced. The selected element dissolves in the aluminum matrix as a single substance to thereby strengthen the matrix. Effects of the multi-element intermetallic compound cannot be expected when the content of each element is less than 0.01%. When the content of each element exceeds 3%, the multi-element intermetallic compound is excessively coarsened such that the physical properties of the bearing alloy are degraded and plastic workability of the bearing alloy such as rolling is degraded. A preferable content ranges between 0.2 and 2%.